Grinding to Size.

by Tom RyanPrinted in Reflections: April, 2009.

I’ve always shied away from making refractor lenses, because the thickness
requirements scared me. For a lens, the optical
designer would always specify the thickness of the lens, in addition to
its radius of curvature, surface finish, surface
accuracy, wedge, etc. Mirrors seemed to me to be a lot easier to make.

Since I’ve been designing optics, I’ve come to understand that
lens thickness is the most error-tolerant of all of the things
specified, but I’ve still been reluctant to make lenses. However,
all things come to those who wait.

I was recently asked to grind and polish some filters for a local medical
company. Their existing filters had the wrong
extinction values, and grinding them thinner was the cheapest way to get
the desired values. Unfortunately, the filters
were composites made of three cemented glass layers, and I had to grind
down and polish one of the glass layers to the
thickness of a piece of paper (0.003”), without breaking through the
optical cement to the dielectric coating, or chipping
or breaking that thin layer of glass.

Here is how I did it.

I first machined an aluminum block to hold the filters in place and at
a common height, since in optics, success or failure
often hinges on good tooling. The filters rested on accurately machined
ledges at equal depths. They have through holes
under them, so their individual thicknesses can be monitored during grinding,
and so wedge can be controlled. The
thickness tolerance on the filters was +/- 0.0004”.

I then cut some glass blockers on a diamond saw, and used optician’s
cement to glue them to the machined aluminum
puck. The glass blockers keep the edges of the filters from turning down.
In the above picture, a filter appears in the
middle of the puck.

Next, the filters were glued in and were ground against a piece of scrap
float glass, which is quite flat from the manufacturer.

The critical thing to know when grinding off 0.122” glass to a tolerance
of +/-.0004” is how much glass is ground off by
each grade of abrasive. The answer to that question involves working backwards
from our desired thickness.

The idea of starting with a coarse abrasive is to be able to grind fast
enough to do the job economically and in a reasonable
length of time. But coarse grinding creates chips and fractures that propagate
into the glass, which have to be removed
by finer abrasives, which in turn leave chips and fractures of their own.
This continues right up to polishing.
Ideally, you’d like to grind as much glass as possible with coarse
abrasive, then switch to polishing. However, you can’t
polish out craters from #80 abrasive in a reasonable length of time. Therefore,
I used a grinding sequence of #120, #320
(nine micron), five micron, and then polished with Zirconium Oxide.

The trick is to leave enough glass at each stage to allow the next stage
to remove the fractures which grinding drives into
glass, and that means knowing how deep the fractures are from each abrasive.
It turns out that the fractures from each
stage are about as deep as the diameter of the abrasive. Polishing compounds
are composed of one micron diameter particles,
and they remove a few microns of glass. Five micron abrasive leaves fractures
about five microns deep, nine micron
leaves nine micron fractures, and so on. I was conservative, and left more
than twice as much glass on the filters at
each stage as was required to clean up the fractures. At 25.4 microns per
0.001”, I left 0.0005” for polishing, 0.001” for
five micron, 0.002” for nine microns, and ground the rest with #120.
It seemed to work.

I poured a pitch lap on an aluminum disk, cut the facets with a couple
of new razor blades (they dull and start to chip the
pitch after cutting about eight channels, and so need to be changed), and
pressed the lap flat with an old fused silica optical
flat.

Ideally, when machine polishing with the optics on top, the lap should
be 6/5 the diameter of the optic. In this case, I
didn’t have a ready-made tool of the correct size, and the lap I had
was considerably bigger than optimum. When this
happens, you have to start polishing with a lap that is already in the
desired shape. In hand polishing, the lap is often the
same size as the optic, and working them together causes the lap to take
on the correct shape. However, when the sizes
are very different, you have to rely on some other method for forming the
lap. Hence the optical flat.

Pitch is soft, and on a microscopic (optical) level, it very much resembles
the seat cushions on a couch. If your optic
doesn’t bridge the entire couch, it will locally sink into the cushions,
and the edges of the glass will get rounded off as
the optic moves across the pitch. That is the reason for having a glass
blocker surround the filters, and for flattening the
pitch by frequently pressing the optical flat onto it during polishing.

The machine polished out the filters in about twenty minutes. If I’d
known they were this fast, I’d have bought a machine
just to polish my first 12.5” mirror.

When the polished filters came off the machine, I checked them on a Zygo
interferometer. They were flat all the way
across to about 1/10 wave. However, they were still 0.0005” too thick
(I had overestimated how much material polishing
removed. It was, after all, the first time I tried this), so I placed a
couple weights on the polishing arm, cranked the
speed up, and let it go for another 40 minutes.

At the end of that time, they were at the correct thickness, but their
overall surface flatness had dropped to 1/4 wave.
(High pressure plowing had rounded the edges.) This is a good argument
for keeping good records, or for letting someone
else do the job first. However, since 1/4 wave was about four times better
than the specifications required, the customer
was quite happy with the results.